Coercivity is the demagnetization force required to reduce polarization or induction to zero.
It is a is a measure of the ability of a ferromagnetic material to withstand an external magnetic field, without getting magnetized.
Coercivity, also called the magnetic coercivity, coercive field or coercive force, is a measure of the ability of a ferromagnetic material to withstand an external magnetic field without becoming demagnetized. An analogous property in electrical engineering and materials science, electric coercivity, is the ability of a ferroelectric material to withstand an external electric field without becoming depolarized.
Coercivity in a ferromagnetic material is the intensity of the applied magnetic field required to reduce the magnetization of that material to zero after the magnetization of the sample has been driven to saturation. Thus (normal) coercivity measures the resistance of a ferromagnetic material to becoming demagnetized. Coercivity is usually measured in oersted or ampere/meter units and is denoted HC. A larger value is the intrinsic coercivity HCi which does not take into account the negative vacuum permittivity contribution to the magnetic field B, only considering the magnetization. There may be ambiguity between the two concepts, and especially for strong magnets the difference becomes significant. The strongest rare earth magnets lose actually almost none of the magnetization at HC, and the reduction of the field to zero is almost equal to the contribution of the vacuum permittivity to the magnetic field. The normal coercivity is used to calculate the magnetomotive force, while the intrinsic coercivity is used to assess the sensitivity to demagnetization.
Ferromagnetic materials with high coercivity are called magnetically hard, and are used to make permanent magnets. Materials with low coercivity are said to be magnetically soft. The latter are used in transformer and inductor cores, recording heads, microwave devices, and magnetic shielding.